In the course of the past few years, the problem of removing heat resulting from operation of electronic devices has grown from being an important concern to becoming a widely recognized bottleneck that limits further progress of high performance microelectronics. Excessive heating affects not only the performance, but also the reliability of computer chips. With a continuing increase in levels of integration and the introduction of new chip and interconnect architectures, the challenge of microelectronics cooling has reached a new heights. Not only have background heat fluxes begun to reach record high values (˜100 W/cm2), but “hot spots” are observed where local heat fluxes exceed several times that found at the background. The air-cooled heat sink has been and remains the main workhorse of the electronic cooling industry. The simplicity and low cost of operation combined with abundance and environmental friendliness of the coolant makes air-cooling uniquely appealing as a first-to-go-approach to thermal management. To overcome the inferiority of air as a heat transfer fluid, over the years the design of air-cooled heat sinks evolved to a staggering level of sophistication with a main goal of providing the highest possible surface area for convective heat transfer in a smallest package and with the lowest possible pressure drop (pumping power) requirement. It should be noted that despite an increasing interest and push towards adaptation of liquid cooling, the air-cooled heat sinks will never be destined to disappear from the research landscape. This is simply because ultimate heat rejection to ambient environment, even in the case of liquid cooling or refrigeration, occurs at the liquid chiller/condenser with an air side of the heat exchanger often defining the overall system size and performance. Finally, to push the limits of air-cooled heat sinks an increased attention has been recently given to two important practical aspects of heat sink design and operation.
First is an issue of the coolant bypass when air introduced into the heat sink avoids traveling through a finned (i.e., active heat transfer) zone, but instead takes the path of minimal hydraulic resistance around and above the heat sink. This scenario has been recently evaluated showing that air bypass results in a rather dramatic increase in the heat sink thermal resistance, which more than doubles with an increase in the number of fins. The second important design aspect concerns evaluation of heat sink performance normalized by the heat sink size and weight. An increase in dissipated heat loads translates into a need for greater heat transfer area, and thus bigger and heavier heat sinks.
Despite technological maturity of air cooling, the art and science of air-cooled heat sink design continues to blossom with innovative ideas pushing the boundaries of performance envelope to their new heights. The most promising avenue for innovation appears to be in exploring and exploiting various methods of active augmentation targeting the two performance-limiting factors—the air throughput enabled by a fan and effective heat transfer coefficients. Synthetic jets and piezoelectric fans are two recent examples of the successful attack on a problem of the air throughput and limited heat storage capacity of cooling air. Specifically, significant performance improvements have been realized by increasing ambient “cold” air delivery, enhanced mixing, and “warm” air rejection using active (actuator-driven) devices: forced flexing of a perforated diaphragm in the case of synthetic jet and piezo-driven flapping of a blade in the case of the piezoelectric fan. This is done in combination with the extended heat transfer surfaces provided by the conventional heat sinks, and with no increase in pressure drop (pumping power) penalty. Another recent interesting idea to augment air throughput through the heat sink exploits gas ion generation by emission from field-enhancing nanostructures, resulting in the microscale ion-driven air flow.
In complimentary efforts, important advances are being made in developing means for enhancing the convective heat transfer coefficient of air cooling. In particular, new twists on a general idea of gas assisted evaporation cooling (Sherwood, G. and Cray, S., “Gas-liquid forced turbulence cooling”, U.S. Pat. No. 5,131,233, 1992) have been recently described. In one approach, called perspiration nanopatch, enhanced evaporation from a capillary-confined thin liquid film subjected to a high velocity dry gas (air) streaming is exploited (Fedorov, A. G., “Nano-Patch Thermal Management Devices, Methods, and Systems”, U.S. patent application having Ser. No. 11/748,540), allowing for dissipation of heat fluxes approaching 500 W/cm2.
In another approach, surface of the heat sink is modified with a sorption material and is cyclically exposed to cold/dry or warm/wet air streams, resulting in thermo-chemical (desorption-based) enhancement of total dissipated heat fluxes as compared to the air cooling alone (Launay, S., Fedorov, A. G., and Joshi, Y., “Thermal Management Devices, Systems, and Methods”, U.S. patent application having Ser. No. 11/867,070).
Finally, it has been recently shown that forced convective liquid cooling can be utilized in combination with air cooling in a hybrid heat sink configuration to provide for synergetic heat removal at different rates from different domains of the microprocessor with the possibility of internal regeneration of the liquid coolant via heat exchange with air (Fedorov, A. G., “Fluid-to-Fluid Spot-to-Spreader Heat Management Devices and Systems and Methods of Managing Heat”, U.S. Patent App. having Ser. No. 60/954,360).